Abstract During embryonic development, the epidermis transforms from a single cell layer to a stratified, fully differentiated tissue that forms a barrier to the external environment. Stratification is driven by proliferation in both the basal cell layer and in a transient transit-amplifying cell population called intermediate cells. These cells express differentiation markers yet remain mitotic, and are hypothesized to promote tissue expansion. However, the roles for intermediate cells have never been directly addressed. Neither do we understand how these cells remain proliferative in an environment that normally promotes differentiation. Here we take advantage of novel mouse lines that we have developed to address roles of intermediate and spinous cells in the expansion and morphogenesis of the epidermis, and to determine how these cells remain mitotically active when not attached to the underlying basement membrane. These studies are expected to identify pathways to promote tissue expansion/maturation for pre-term infants and for regenerative medicine, as well as provide novel mechanistic basis for signaling between epidermal cell layers.